1. Field of the Invention
The present invention relates to apparatus and method for the detection of abnormal conditions in well drilling operations by monitoring the drilling mud used in the drilling operations. The invention also relates to a novel system of handling the mud in carrying out the drilling operations.
2. Description of the Prior Art
In drilling a well by rotary drilling operations, a drilling fluid frequently called a "mud" is pumped down the drill string through the bit and back to the surface through the well annulus. The mud provides several important functions including maintaining a hydrostatic head of pressure, cooling the bit, and removing drilled particles.
In the drilling of wells, it is important to closely monitor drilling operations to detect any abnormal drilling condition that might be encountered. Under normal drilling and circulation conditions, formation fluid will not enter the well and mud will not be lost to the formation. The volume of the mud system will thus remain substantially constant. (It is recognized that a small amount of fluid will be lost through normal filtration, evaporation, and leakage. For purposes of monitoring a mud system, however, the volumes resulting from these losses may be ignored.)
An abnormal drilling or circulation condition generally results in a change in the mud volume. A particularly hazardous abnormal condition occurs when a high pressure zone is encountered. If the mud in the well does not provide a sufficient hydrostatic pressure in this zone, formation fluids will enter the well and could result in a blowout. The entry of gas is manifested by the forcing of an equivalent amount of drilling mud from the wellbore into the surface mud tanks or pits. This condition is referred to as a "kick". The initial entry may cause only a slight change in the volume of mud in the tanks; however, as the gas rises it increases its volume as the pressure of the column of mud is reduced forcing more mud out of the wellbore and allowing the entry of more gas into the bore until a blowout occurs.
When a kick is encountered, the control of a well is best maintained by acting early. However, prior art detection techniques which rely on changes in pit mud level frequently do not permit early detection of a kick. The accuracy with which the mud level can be measured is a function of the sensitivity of the fluid level detection method. For example, in a conventional system of four 8 ft by 40 ft tanks, a 9.5 barrel change in the tanks results in only a 0.5 inch change in the mud level.
Another hazardous drilling condition that must be closely monitored is that of lost circulation. If the hydrostatic pressure imposed by the mud on a subterranean formation is too great, the formation may be fractured causing mud to be lost to the formation. This, as in the blowout condition, is generally indicated by change in mud pit level.
A conventional mud volume totalizer marketed by Martin Decker Company is shown on page 4510 of the Composite Category of Oil Field Equipment and Services, Volume 3, 1978-79, published by World Oil. Other mud level detection devices are disclosed in U.S. Pat. Nos. 3,086,397 and 3,608,653. As mentioned previously, these devices are used in conventional systems and are not sensitive enough to provide early and reliable detection of abnormal conditions.